Magneto inductive flow measuring devices utilize the principle of electrodynamic induction for volumetric flow measurement are known from a large number of publications. Charge carriers of the medium moved perpendicularly to a magnetic field induce a measurement voltage in measuring electrodes arranged essentially perpendicularly to the flow direction of the medium and perpendicularly to the direction of the magnetic field. The measurement voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube, and is, thus, proportional to the volume flow rate. If the density of the medium is known, the mass flow in the pipeline, respectively in the measuring tube, can be determined. The measurement voltage is usually registered across a measuring electrode pair, which is arranged relative to the coordinate along the measuring tube axis in the region of maximum magnetic field strength and where, thus, the maximum measurement voltage is to be expected. The electrodes are usually galvanically coupled with the medium; there are, however, also magneto inductive, flow measuring devices with contactless, capacitively coupling electrodes.
The measuring tube can, in such case, be manufactured either from an electrically conductive, non-magnetic material, e.g. stainless steel, or from an electrically insulating material. If the measuring tube is manufactured from an electrically conductive material, then it must be lined in the region coming in contact with the medium with a liner of an electrically insulating material. The liner is composed, depending on temperature and medium, for example, of a thermoplastic, a thermosetting or an elastomeric, synthetic material, or plastic. There are, however, also magneto inductive, flow measuring devices, which have a ceramic lining.
Known from published application US 2004/0149046 A1 is an electromagnetic flow measuring device having a measuring tube, which has a planar area and an otherwise cylindrical surface. Arranged on the planar area is a pole shoe of a magnet system. In such case, the pole shoe, the inner core and the coil are secured on an outer core (for flux guide back) by a screwed connection against slipping. Unfavorable in the case of this embodiment is the conventional flush, shape interlocking bearing of the pole shoe on the measuring tube, since this places high requirements on tolerances. Problematic is the mechanically little stable affixing of the pole shoe on the measuring tube only via the external core. This can lead in the case of vibrations to a degrading of the measurement signal.
U.S. Pat. No. 2,734,380 discloses a magnetic flow measuring device having a measuring tube, in the case of which the pole shoes of two magnet coils lie on a flat area. The two magnet cores are affixed on the measuring tube by externally situated bolts (126 in the patent, FIG. 3). Disadvantageous in the case of this embodiment is that several bolts are necessary and that, in the case of a non-symmetric tightening torque, additional measurement deviations can occur.
European Patent No. EP 1 674 836 A1 discloses a magneto inductive, flow measuring device with a coil and at least two components connected releasably with one another, wherein the component lies flat on a region of a right angled measurement line and is held by an external tube on this measurement line. Tolerances in the outer diameter of the measuring tube are compensated, in such case, by a flexible piece of sheet metal, which presses the core and, via the core, the pole shoe against the measuring tube. Disadvantageous in the case of this embodiment is that the force, with which the sheet metal presses on the core, is relatively small relatively due to the thinness of the sheet metal, a feature which can disadvantageously influence the sensitivity of the measuring device and its stability e.g. in the case of vibrations.
U.S. Pat. No. 5,751,535 discloses an electronic flow measuring device having a cylindrical measuring tube and pole shoes of a magnet bearing thereon. Disadvantageous in the case of these variants of embodiment are especially the aperture angle of the pole shoe, which depends on the tube tolerances, since the bearing points are not defined via a planar, machined area, but, instead, via the contact of the pole shoes on the periphery of the measuring tube. Depending on diameter or deviation from ideal roundness of the measuring tube, the bearing points of the pole shoe change on the measuring tube. When, in the mounting, the pole shoe is pressed against the measuring tube, the legs bend dependent on the measuring tube geometry and the applied force. This can lead to an additional measurement deviation.
German Patent No. DE 103 06 522 discloses a system for the efficient and guided mounting and affixing of a magnet system on a pipe of rather small diameter. This is especially achieved by an advantageous shaping of the coil form, the coil core and the field guide-back path, and is less advantageous in the case of greater tube diameters.
German Patent No. DE 35 45 155 discloses, among others things, the affixing of a magnet system on a housing composed of a number of parts, instead of directly on the measuring tube (see FIG. 3 in the patent). Disadvantageous here is that, depending on the dimensions and tolerances of the components, a separation between pole shoe and measuring tube can occur or the housing does not fit over the core.
An electrode can be subdivided essentially into an electrode head, which comes in contact at least partially with a medium flowing through the measuring tube, and an electrode shaft, which is situated almost completely in the wall of the measuring tube.
The electrodes are, besides the magnet system, the central components of a magneto inductive, flow measuring device. In the embodiment and arrangement of the electrodes, attention is to be paid that they can be assembled in the measuring tube as simply as possible and that subsequently in measurement operation no sealing problems occur; moreover, the electrodes should provide a sensitive and simultaneously low-disturbance, registering of the measurement signal.
Besides the measuring electrodes, which serve for registering a measurement signal, often additional electrodes are installed in the measuring tube in the form of reference- or grounding electrodes. These serve to measure an electrical reference potential or to detect partially filled measuring tubes or to register the temperature of the medium by means of an installed temperature sensor.